The present invention relates to thermal paste and particularly relates to a liquid metal matrix thermal paste containing fine thermally conducting particles dispersed in a low melting temperature liquid metal matrix and having high overall thermal conductivity. The thermal paste is used to form a thermal joint between an electronic component, such as a chip, and a cooling system.
Modern very high performance electronic systems often require a high density of chips having many high power gates. These electronic systems require cooling a high power density through a limited temperature drop from a device junction to a cooling system. In order to achieve the cooling requirement, the thermal joint from the chip to the cooling system must possess a high thermal conductivity. The thermal paste described hereinafter provides the highest available thermal conductivity thermal joint between a chip or component and the cooling system obtained to date.
In practical applications, the joint, in addition to providing high thermal conductivity, must compensate for certain manufacturing tolerances inherent in any electronic assembly. For instance, when assembling chips using the so-called controlled collapse circuit connector technique (hereinafter referred to as C4), as a result of variations in the size and shape of the solder ball arrays used to connect the chip to a printed circuit board, there are significant variations in chip height and tilt relative to the printed circuit board. The electrical connections, the C4 solder balls, are very fragile and the thermal joint must permit a certain amount of differential motion of the individual chips forming an electrical system assembly. Also, manufacturing tolerances cause variations of the gap between the chip and the cooling system. The combination of geometric variations and tolerances conflict with the goal of achieving good thermal conduction by causing a very thin paste layer to be manifest between certain of the chips and the cooling system. Despite the difficulty encountered, good thermal conduction is still achievable by using a paste having high bulk thermal conductivity properties.
Using known conventional thermal paste has provided moderately good heat transfer plus moderate compliance. A commonly used paste contains a mixture of zinc oxide in mineral oil. Such pastes have an upper limit of thermal conductivity. Also, the liquid and particles tend to phase separate after many power on-off cycles. The conventional pastes rely upon the perculation of oxide particles in a low conductivity oil matrix for thermal conductivity. The use of a low conductivity oil matrix is the primary limiting factor in achieving high thermal conductivity. The present invention provides for a high thermal conductivity of both the liquid matrix and the dispersed particles.
There have been many proposals to use liquid metal thermal joints, particularly mercury, which may be harmful both to humans and electronic circuits. However, confining the liquid metal has been prohibitively difficult and requires a reduction of the thermal conductivity. For instance, U.S. Pat. No. 4,092,697 teaches a thermal joint pillow with a plastic film skin and macroscopic filling of liquid metal. Also, U.S. Pat. No. 4,323,914 teaches the coating of the entire chip with a parylene film coating and then adding a macroscopic metal joint to the cooling cap. Both of these patents degrade the thermal conductivity of the joint by including a poor thermal conductivity plastic film.